Not by the dozen, but when I buy them, I buy several from the section at Target where they have all the other harmless kindergarden items - fat crayons and pencils as well as round tip, dull-bladed plastic-handled scissors.

Though they are not usually "voluntarily surrendered" in the US, I too have encountered problems in Europe. So I just have spares at home to replace the ones taken, as they are not expensive but they are useful to have. And yes, I have a tape measure in the same kit as the scissors just in case someone wants to argue about whether the scissors are longer than 2 inches or 4 inches. Also tiny nail clippers that will do to trim a loose thread when the scissors have been taken.